Tissue products, such as facial tissues, paper towels, bath tissues, napkins, and other similar products, are designed to include several important properties. For example, the products should have good bulk, a soft feel, and should have good strength and durability. Unfortunately, however, when steps are taken to increase one property of the product, other characteristics of the product are often adversely affected.
Tissue products are made via one of two primary tissue manufacturing processes: conventional wet press (CWP) and through-air drying (TAD). In CWP, the tissue is formed on a forming fabric from either a suction breast roll or twin wire former and the embryonic web is transferred to a papermaking felt and dewatered by pressing with one or two pressure roll nips against the surface of a large steam heated cylinder called a Yankee dryer. The pressing process also assists in transfer of the sheet to the Yankee dryer surface. An adhesive solution is sprayed on the dryer surface prior to the sheet transfer in order to provide good bonding between the sheet and the dryer surface. The sheet is removed from the Yankee surface by a doctor blade in the creping process. In the TAD process, the sheet is formed on a forming fabric and transferred to one or more other fabrics as it is dewatered to a consistency of 25 percent or higher. After the initial dewatering the sheet is dried while in contact with the fabric by blowing hot air through the fabric until the consistency is 40 percent or higher. In conventional through-air dried processes, the through-air dried web is adhered to a Yankee dryer and creped. A roll may be present at the point of transfer to assist in the transfer of the web from the drying fabric to the Yankee dryer but absent the presence of high pressure used to dewater the web in the CWP process. Alternatively TAD tissue may be prepared without creping where foreshortening of the web occurs with a differential velocity transfer of the wet laid web from the forming fabric to a substantially slower moving, open mesh transfer fabric. Thereafter the web is dried while preventing macroscopic rearrangement of the fibers in the plane of the web. The web is then dried on a fabric in the through-air dryer to a consistency of 90 percent or higher and wound. No Yankee dryer is used in the uncreped through-air dried (UCTAD) process. Through-air dried tissue products are typically associated with higher quality tier tissue products than conventional wet pressed products due to their higher bulk and greater absorption capacity.
To achieve the optimum product properties, tissue products are typically formed, at least in part, from pulps containing wood fibers and often a blend of hardwood and softwood fibers to achieve the desired properties. Typically when attempting to optimize surface softness, as is often the case with tissue products, the papermaker will select the fiber furnish based in part on the coarseness of pulp fibers. Pulps having fibers with low coarseness are desirable because tissue paper made from fibers having a low coarseness can be made softer than similar tissue paper made from fibers having a high coarseness. To optimize surface softness even further, premium tissue products usually comprise layered structures where the low coarseness fibers are directed to the outside layer of the tissue sheet with the inner layer of the sheet comprising longer, coarser fibers.
Unfortunately, the need for softness is balanced by the need for durability. Durability in tissue products can be defined in terms of tensile strength, tensile energy absorption (TEA), burst strength and tear strength. Typically tear, burst and TEA will show a positive correlation with tensile strength while tensile strength, and thus durability, and softness are inversely related. Thus the paper maker is continuously challenged with the need to balance the need for softness with a need for durability. Unfortunately, tissue paper durability generally decreases as the average fiber length is reduced. Therefore, simply reducing the pulp average fiber length can result in an undesirable trade-off between product surface softness and product durability.
Besides durability long fibers also play an important role in overall tissue product softness. While surface softness in tissue products is an important attribute, a second element in the overall softness of a tissue sheet is stiffness. Stiffness can be measured from the tensile slope of stress-strain tensile curve. The lower the slope the lower the stiffness and the better overall softness the product will display. Stiffness and tensile strength are positively correlated, however at a given tensile strength shorter fibers will display a greater stiffness than long fibers. While not wishing to be bound by theory, it is believed that this behavior is due to the higher number of hydrogen bonds required to produce a product of a given tensile strength with short fibers than with long fibers. Thus, easily collapsible, low coarseness long fibers, such as those provided by Northern Softwood Kraft (NSWK) fibers typically supply the best combination of durability and softness in tissue products when those fibers are used in combination with hardwood Kraft fibers such as Eucalyptus hardwood Kraft fibers. While Northern Softwood Kraft Fibers have a higher coarseness than Eucalyptus fibers their small cell wall thickness relative to lumen diameter combined with their long length makes them the ideal candidate for optimizing durability and softness in tissue.
Unfortunately, supply of NSWK is under significant pressure both economically and environmentally. As such, prices of NSWK fibers have escalated significantly creating a need to find alternatives to optimize softness and strength in tissue products. Another type of softwood fiber is Southern Softwood Kraft (SSWK) widely used in fluff pulp containing absorbent products such as diapers, feminine care absorbent products and incontinence products. Unfortunately while not under the same supply and environmental pressures as NSWK, fibers from SSWK are too coarse for tissue products and are unsuitable for making soft tissue products. While having long fiber length, the SSWK fibers have too wide a cell wall width and too narrow a lumen diameter and thus create stiffer, harsher feeling products than NSWK.
The tissue papermaker who is able to obtain pulps having a desirable combination of fiber length and coarseness from fiber blends generally regarded as inferior with respect to average fiber properties may reap significant cost savings and/or product improvements. For example, the papermaker may wish to make a tissue paper of superior strength without incurring the usual degradation in softness which accompanies higher strength. Alternatively, the papermaker may wish a higher degree of paper surface bonding to reduce the release of free fibers without suffering the usual decrease in softness which accompanies greater bonding of surface fibers. As such, a need currently exists for a tissue product formed from a fiber that will improve durability without negatively affecting other important product properties, such as softness.
Outside of Northern and Southern softwood pulp fibers very few options exist for papermakers when selecting long fibers. Bamboo fibers have been used in paper for many years primarily in India and China. Long and short fiber bamboo species are reported. Bamboo fibers have been used to replace wood fibers in blended, conventional wet pressed tissue products in India and China. Unfortunately commercially available bamboo pulps comprise a mixture of species of bamboos including long and short fibers. Length weighted average fiber lengths are significantly shorter than northern and southern softwood pulp fibers. In addition, the bamboo pulps contain a high portion of primary fines in the form of parenchyma cells. It is well known that the presence of high levels of fines in tissue products create a decrease in surface softness and an increase in stiffness of tissue sheets. Thus, tissue sheets comprising high amounts of bamboo fibers tend to be less soft than those comprising wood fibers. Moreover, given the relatively short fiber length and high level of primary fines, bamboo pulp fibers would not be predicted to provide good durability or softness properties to premium tissue products when specifically replacing wood pulp fibers. Generally, bamboo fibers have coarseness and fines that are too high to replace hardwood fibers and an average fiber length that is too short and cell walls that are too thick to replace softwood fibers.